1. Field of the Invention
The present invention relates to improvements in DC/DC switched mode power supply topologies. More particularly, the invention relates to improvements particularly suited for multiple module topologies. In particular, the present invention relates specifically to a coupled inductor output filter allowing load sharing between multiple power supplies without inter-module feedback between the power supplies.
2. Description of the Known Art
As will be appreciated by those skilled in the art, power supply topologies are known in various forms. Patents disclosing information relevant to power supplies include: U.S. Pat. No. 4,703,409, issued to Spreen on Oct. 27, 1987 which is entitled Coupled power supply inductors for reduced ripple current; U.S. Pat. No. 7,317,305, issued to Stratakos et al. on Jan. 8, 2008 which is entitled Method and apparatus for multi-phase dc-dc converters using coupled inductors in discontinuous conduction mode; U.S. Pat. No. 7,449,867, issued to Wu et al. on Nov. 11, 2008 which is entitled Multi-phase buck converter with a plurality of coupled inductors; and U.S. Pat. No. 7,498,783, issued to Johnson on Mar. 3, 2009 which is entitled Extending the continuous mode of operation for a buck converter. Patents and/or applications relating to coupled inductors also include the basic electrical components of the present design as noted by United States Patent No. 2009/0179713 filed by Zeng et al. published on Jul. 16, 2009 entitled Low pass filter incorporating coupled inductors to enhance stop band attenuation. Each of these patents and applications is hereby expressly incorporated by reference in their entirety.
Increasing power density and efficiency, reducing size and weight, and introducing standardization of electronics systems are all goals of the DC/DC converter power electronics community. Standardized building block modules can decrease cost and deployment time while improving capability and reliability while enabling seamless scalability. The modular converter concept is an ideal solution to diminish time and expenses associated with the implementation of typical converters. However, there are several inherent and practical challenges when attempting to develop a truly modular system using DC/DC converters as power building blocks. Next, a brief discussion of the inherent load imbalance phenomenon encountered in paralleled DC/DC converters is presented.
Inherent Problems (Power Plane Analysis)
Two ideal voltage sources (i.e., zero internal impedance) connected in parallel will share current equally, if and only if their individual voltages are perfectly matched. On practical voltage sources such as DC/DC converters with closed control loops, small voltage differences will allow for one of the sources (i.e., DC/DC converter) to “take over” or deliver most of the system current. FIG. 2 shows a two, parallel-input, parallel-output (PIPO) arrangement.
FIG. 3 is an oscilloscope capture of the output voltage (Vout) and the individual output currents of each module (10 and 11). In this particular case, module M0 is providing virtually all the current to the load while M1 is not outputting current.
The topologically inherent power sharing problem can be explained with the output power plane analysis. The output power plane is the graphical representation of the output current versus the output voltage of a particular converter [1].
FIG. 4 is an idealized power plane representation of the system in FIG. 2. Vp is the overall system operating voltage and Vout0 and Vout1 are the characteristic standalone output voltages of M0 and M1 respectively. Good voltage converters act as a voltage source with a small output impedance (i.e., the negative reciprocal of the slope of the lines seen in FIG. 4). The difference in output voltages originates from the intrinsic component differences of each module. The converters can be very similar, but they will never be exactly alike. This means that in any multi-module PIPO configured system there is always one converter that operates at a slightly higher output voltage than the others. This fact is important in understanding the effects of traditional proportional integral (PI) control loops found in most DC/DC converters.
As shown in FIG. 4, the module that operates at a slightly higher output voltage will provide all the current. This is derived from the small output impedance characteristic of a DC/DC converter operating under output voltage closed loop condition. In general the control PI loop under steady-state conditions performs small corrections in order to minimize the steady state error. This is the desired behavior for a standalone module. However, when two modules are PIPO connected, this typical behavior is counteractive to achieving even current sharing.
Literature involving prior art systems also includes Glaser, J. S.; Witulski, A. F.; “Output plane analysis of load-sharing in multiple-module converter systems” Power Electronics, IEEE Transactions on Volume 9, Issue 1, January 1994 Page(s):43-50 which indicates that discontinuous conduction mode converters (i.e., the output filter inductor current is depleted during each switching cycle) offer a favorable output impedance behavior for parallel operation. However, this output impedance behavior is only applicable for a limited range of output current. There is a maximum load current at which the converter leaves the discontinuous conduction mode and enters continuous conduction mode. In order to overcome this problem, a new power filter topology with inherent power sharing capabilities has been developed.
From these prior references it may be seen that these prior art patents are very limited in their teaching and utilization, and an improved filter topology is needed to overcome these limitations.